1. Field of the Invention
The present invention relates to a characteristic image extraction method and an ophthalmologic apparatus, in particular, a characteristic image extraction method and an ophthalmologic apparatus which involve a devised tracking function working when a fixation index is moved.
2. Description of the Related Art
At present, various optical equipments are used as ophthalmologic apparatuses or ophthalmic equipments. Of those, used as an optical equipment for observing an eye are various equipments including an anterior ocular segment photographing device, a fundus camera, a confocal scanning laser ophthalmoscope (SLO), and an optical coherence tomography (OCT) apparatus that is an optical tomographic imaging apparatus using optical interference due to low coherence light. In particular, the OCT apparatus using the optical interference due to the low coherence light is an apparatus for obtaining a tomographic image of a fundus with high resolution, and is currently on the way to becoming an indispensable apparatus in outpatient clinics specialized in retinas.
The OCT apparatus is an apparatus for applying low coherence light to a sample represented by a retina and measuring reflected light from the sample with high sensitivity by using an interferometer. Further, the OCT apparatus can obtain a tomographic image by scanning the low coherence light over the sample. In particular, the tomographic image of a retina is widely used in ophthalmic diagnosis.
In recent years, the OCT apparatuses conventionally of a so-called time domain method, in which a tomographic image is acquired by performing scanning along an optical path length of a reference beam in each tomographic acquisition position, have made a shift to OCT apparatuses of a Fourier domain method that eliminates the need for the scanning along the optical path length of the reference beam in each tomographic acquisition position. Therefore, it is now possible to acquire the tomographic image with faster speed than before. The increase in the speed allows three-dimensional image taking of the fundus, which has been difficult with the time domain method, to be carried out.
However, in the three-dimensional image taking, two-dimensional scanning of the fundus is necessary even with the Fourier domain method, and hence an influence of an involuntary eye movement of an eye to be inspected cannot be ignored. The influence of the eye movement can be reduced by detecting the eye movement at the time of image taking and correcting a low coherence light beam scanned over the retina in synchronization with the detected eye movement. The detection of the eye movement can be realized by taking a fundus image in parallel with the image taking performed by the OCT apparatus and detecting displacements of characteristic points within the fundus image.
Further, in some of the ophthalmologic apparatuses or ophthalmic equipments, in particular, equipments for taking a fundus image, an image is in some cases acquired by greatly changing a fixation point of the eye to be inspected according to an imaging target region of the eye to be inspected. A fixation index or a fixation lamp is used to change the fixation point of the eye to be inspected. Also in the OCT apparatus, when a photographing position is to be greatly changed, the fixation point of the eye to be inspected is caused to change by moving the fixation index. This greatly changes a range of the fundus that can be observed through a pupil. By appropriately changing the position of the fixation index, the imaging target region can be guided to a position that can be observed.
Japanese Patent No. 3,708,669 discloses a fundus imaging apparatus configured as follows. That is, when the fixation index is moved in order to change the imaging target region during the taking of the fundus image, a relationship between an optical axis and a pupil position is changed simultaneously. According to the change, the optical axis is automatically adjusted so as to be the center of the pupil.
Japanese Patent No. 3,708,669 discloses automatic adjustment of the change in the relationship between the optical axis and the pupil position, which becomes a problem when a photographing target region is changed by performing guidance with the fixation index.
However, there is no description regarding the characteristic points within the fundus image being used to detect the eye movement. For the fundus imaging apparatus having such a function, nothing is taken into consideration regarding the problem that a region that can be observed changes according to the movement of the fixation index and the characteristic points within the fundus image are displaced from the region that can be observed.